I would think that a rifle leaning against a tree and falling over on to the forest floor should not be a problem at all. I've had that happen and there was no change of zero that I detected.
Modern brand name scopes and mounts are made better than they've ever been and are pretty rugged and it's expected that there will be the occasional bump.
If scopes lost their zero from mild shocks then a lot of hunters would be going back to irons.
